Friday, 29 January 2016

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

There's quite a bit of discussion over at Résonaances (see also the comments) surrounding the Davis-Fairbairn-Heal-Tunney paper proposing an underestimated systematic in the background parameterisation used in the ATLAS diphoton analysis. This (and related) discussion looks to have aided (according to the acknowledgments) the preparation of another paper from Bradley Kavanagh, which seems to clarify the issue. In that paper it is written:

Davis et al. introduce a different possible parametrisation for the background (which was also validated by a Monte Carlo study) and find that the significance of the excess is further reduced with respect to the k = 1, fixed-N case. However, the empty bins at high mγγ were not included in that analysis, leading to a background fit which overestimates the high mγγ event rate. Indeed, using the Davis et al. background parametrisation (with free normalisation) in this analysis gives a local significance of 3.8σ for a free-width resonance. This does not discount the possibility that exploring a wider range of possible background functions may impact the significance of the 750 GeV excess, but the correct constraints from the entire range of mγγ should be taken into account.

A few-interesting-papers appeared concerning baryonic effects on the local dark matter velocity distribution, of interest for interpreting direct detection experiments (see Matthew Buckley's blog for a write-up of one of them). Each of the papers takes a number of simulated Milky Way-like galaxies and looks at the dark matter distribution at Solar radius. Naturally, due to the small number of simulated galaxies, the papers reach slightly different conclusions. What is clear, though, is that there are significant uncertainties in both the local density and the local velocity distribution, which means that the usual direct detection limits you see drawn on e.g. σSI versus mχ space should be taken with a small grain of salt, since they assume the standard halo model. Also of note is that these effects alone cannot ameliorate tension with the DAMA/CoGeNT events. Further work in this area will be interesting to follow as additional (and more detailed) simulations become available.

I personally think that it is a good exercise for the hep-ph community to ask the question, if it is real, then what could it be? At least for the scientifically motivated reason that extra predictions are generally made which might be tested, and these predictions could in principle serve as a guide to tell experimentalists where to probe nature next (in the case that this turns out to be real). It is also sensible to collectively gather ideas which might help to fit the thing into a bigger picture. Unfortunately these good scientific motivations are confounded by citation-chasing, repetition, ill-motivated "Hail Mary" models, repetition, repetition, etc. We must also be aware of our (unscientific) cognitive bias toward fluctuations from the mean: given the statistical significance of the signal, is all this work sufficiently scientifically motivated? This is an interesting question, if rather academic... it is naive to think that scientists are (or even should be) motivated by purely scientific considerations.

Anyway, the time should come for we as a community to sit back and take stock. The problem then is, among the noise, how to reduce the growing theory-space to a set of distinct generic predictions. I am considering pursuing this in the form of a wiki (or similar) as an experiment in large-scale collaboration; the idea would be to produce a summary document which represents a balanced cross-section of hep-ph ideas on this thing (with no cap on author count). The difficulties include the administrative one of keeping such a project economic and efficient, but also keeping a fair balance and controlling the (possibly inevitable) politics involved. If you have ideas or would like to get involved in such a project, please leave a comment or send me an email, so that I may gauge the interest in such a thing...

There is not too much more to say except that there are myriad explanations for this possible signal, and I think it is sensible to be ready if it does turn out to be real. That being said, it would take a brave person to claim that the odds are in its favour...

Before Christmas we finished up on a fun project: "Plasma dark matter direct detection." The paper concerns what is a rather under-appreciated and somewhat generic point about self-interacting dark matter models and direct detection experiments. The logic goes like this:

(1) If dark matter is self-interacting and capable of giving a direct detection signal, then some amount will be captured within the Earth. (2) The annually varying dark matter wind will interact with this captured dark matter in a highly non-trivial way. (3) This will result in a complex space- and time-varying dark matter near-Earth environment. (4) The dark matter detector moves through this environment throughout the day/year, and the rate it measures will be a time-average of the local rate along its path through space.

In the well studied WIMP dark matter scenario, there is no spatial dependence of the dark matter distribution near the Earth, and so it doesn't matter where your detector is in space. Our scenario is quite different. Both the dark matter wind speed and the detector's daily path annually modulate due to the Earth's motion around the Sun. These modulations have different phases (155 days vs 115 days). So now you have two sources of annual modulation which, due to the complex dark matter environment, give an annually modulating rate which does not necessarily resemble a sinusoid. The following animations should help to visualise this picture:

These are two simplified captured dark matter scenarios (fully absorbing/reflective) which we considered. The dark matter wind comes in from the left and its speed annually modulates. The direction of the Earth's rotation axis with respect to the wind also annually modulates, and therefore so do the detectors' daily paths: the black, green, orange, red bars represent the location of detectors in Gran Sasso, Kamioka, China Jin-Ping, and Stawell, respectively. Clearly, due to the complex environment, they will measure very different things! This is the qualitative picture; to make quantitative predictions is very difficult. This is why multiple experiments at multiple latitudes will be important for probing this scenario, especially experiments in the Southern Hemisphere (such as Stawell) which inhabit a unique location behind the Earth with respect to the wind.

Lastly, the generic and distinctive prediction of these models is a possibly strong and non-trivial modulation as a function of time of sidereal day (diurnal modulation). A sidereal day is an "astronomical day" slightly shorter than a 24 hour day; there are approximately 366 sidereal days in a year. It is hard to imagine any background process which would modulate with period of one sidereal day. It therefore seems like a very sensible dark matter search to perform in addition to an annual modulation search.

Already in a few previous posts I mentioned the recent XMASS annual modulation search and its possible hint of a modulation signal with opposite sign to that of DAMA. Out of interest, last week I got around to scraping their central values from the data in the backup slides of their TAUP talk [pdf]. Below I present their measurement of rate as a function of time for energy bins summed from 0.5--2.0 keV57Co.

The error bars are statistical only (though they dominate the systematic error) and have been estimated assuming equally spaced bins (which is not exactly correct); these errors are therefore only there to guide the eye and the actual ones would be if anything slightly larger. For interest the sinusoid of best fit, with a phase of 129 (or 311) days, is also plotted.

Their result is clearly intriguing. It looks convincing to me, though one would need another year of data to tell for sure, and it will be interesting to see whether this effect continues in their fiducial volume (this analysis is full volume). What's going on here? It is consistent with a seasonal effect, but with amplitude opposite to that of DAMA. Though possible, if the modulation is due to an environmental effect then at least qualitatively this seems strange, since each of XMASS/DAMA are in the Northern Hemisphere (XMASS at Kamioka 36°N, DAMA at 43°N). The results of the annual modulation experiments sure are puzzling: there are four published now each seeing an effect at some level (though apart from DAMA are statistically weak)...

Time might tell, but a speculative observation: if the XMASS effect is due to a non-trivial dark matter distribution, then the small change in latitude suggests that their signal will almost certainly be accompanied by large diurnal variation. So if XMASS see annual modulation in their fiducial volume, I would be very interested to see their search for a diurnal signal.

The XXII The Cracow Epiphany Conference on Run II LHC Physics (indico) is currently on.

About Me

Jackson Clarke, PhD candidate in phenomenological particle physics at CoEPP, University of Melbourne. Collider phenomenology, neutrino masses, and some naturalness. Science enthusiast, among many other things. Blogging accordingly.

Views are my own. Content very definitely skewed by my own leanings and by papers getting attention. So it goes.